A federal judge ruled Monday that the Army Corps of Engineers failed to fully analyze potential flooding downstream of a planned leg of the Grand Parkway in west Harris County but refused to block the project.

U.S. District Judge Keith Ellison said the four-lane outer beltway, already under construction, should not be stopped because the problems with the federal agency's analysis involves future development and not the project itself. An extended delay could cost the state as much as $21 million, Ellison wrote in a 42-page ruling.

Take another look

Even then, the judge said the Corps must take another look at the flooding consequences of more construction of roads, houses and office and retail buildings that are bound to follow the Grand Parkway upstream of the Addicks dam.

Jim Blackburn, an attorney for the Sierra Club, which sued to block the road over flooding concerns, said he was pleased with the decision because it will require the Corps to take "a hard look" at future development when permitting projects.

"It's a correct decision," he said. "I would have liked to see it go further, but it has potentially far-reaching impacts."

The Sierra Club claimed the 15-mile toll road may increase runoff into Addicks dam, which the Corps has identified as among the nation's riskiest because of the potential harm to low-lying Houston should the 1940s-era structure give way.

The environmental group filed the lawsuit in U.S. District Court in Houston last August after the Corps issued the final permit for the new leg of the long-planned outer beltway. The toll road would run through the Katy Prairie between U.S. 290 and Interstate 10 - a mostly undeveloped area that stores rainwater like a natural sponge.

The Army Corps is both the primary steward of the prairie's wetlands and the agency responsible for dam safety. The agency could not be reached for comment Monday.

'Extremely high risk'

The Corps identified the Addicks and nearby Barker dams as "extremely high risk" in 2009 after finding empty spaces in the foundation beneath the conduits, or tunnels, that control the flow into Buffalo Bayou. At the time, officials blamed the seepage on the increased presence of stored water behind the structures - a result of more people, houses and businesses upstream and downstream of the dams.

Despite the alarm, the corps has said the actual risk of failure is low.